1. Field of the Invention
The present invention relates to a semiconductor apparatus with high integration density, which is capable of effectively cooling a semiconductor element with low radiation efficiency.
2. Description of the Related Art
A conventional semiconductor apparatus comprising mainly a plurality of semiconductor elements mounted on a circuit board such as a printed circuit board or a ceramic board consumes much power and is equipped mixedly with high-power-consuming, highly exothermic semiconductor elements (hereinafter referred to as "exothermic element") and semiconductor elements having less power consumption than the exothermic element and having not so high exothermic properties (hereinafter referred to as "IC element"). Normally, the packages of all exothermic elements are provided with heat sinks such as cooling fins. Recently, the exothermic density has remarkably increased due to an increase in integration of the semiconductor element itself and an increase in density of integration of semiconductor elements. Accordingly, the cooling of the semiconductor apparatus is a serious problem.
An example of a semiconductor apparatus having the conventional cooling structure will now be described with reference to FIG. 1 (plan view) and FIG. 2 (partial cross-sectional view). Some semiconductor apparatuses are combined into a single system having a predetermined function. The system is provided with a cooling apparatus for forcibly producing an air flow. The cooling apparatus introduces an air wind into the system, thereby dissipating heat generated by a semiconductor apparatus situated at a predetermined position within the system, as shown in FIG. 1.
A wiring pattern such as a Cu layer is formed on a major surface of a circuit board 15 such as a printed circuit board (PCB). Semiconductor elements mounted on the circuit board 15 are connected to the wiring pattern. On the circuit board 15, an exothermic element 11 is situated at a position where the air flow can be utilized most effectively. An IC element 12 and peripheral circuits (not shown) are formed on the other region. A cooling fin 13 with high heat radiation properties is attached to the surface of the package of the exothermic element 11 such as a CPU which must be designed most deliberately in terms of thermal properties. The cooling fin 13 receives an air flow 16 produced from a cooling device 20 attached to the system, thus radiating heat from the exothermic element 11. The IC element 12 such as a memory with not so high exothermic properties is not equipped with a cooling fin. As is shown in FIG. 2, in the IC element 12, a semiconductor chip 5 is coated and protected by a resin-mold package 6. The resin-mold package 6 may be replaced by a ceramic package or a laminated-ceramic package. The semiconductor chip 5 is fixed to a chip-mounting portion 7 formed from a lead frame, the chip 5 and leads 8 of the lead frame are connected by bonding wires 9. The semiconductor chip 5, as well as the chip-mounting portion 7, bonding wires 9 and portions of the leads 8, is coated in the package 6. In this prior art, a laminated ceramic package is used for the exothermic element 11.
In the laminated ceramic package, since wiring can be formed on laminated ceramic substrates, multi-layer wiring can be formed and an exothermic element with high integration density can be mounted. The cooling fin 13 is attached to the bottom surface (facing upwards) of the laminated ceramic package 2 formed of, e.g. alumina. The package 2 is made of a laminated ceramic material, and wiring patterns (not shown) are formed between layers of the ceramic material. As the semiconductor elements are integrated at higher density, the number of laminated layers increases. A semiconductor chip 1 is mounted at a center portion of the package 2. The integrated circuit formed within the semiconductor chip 1 and the laminated wiring pattern formed within the package 2 are connected by bonding wires 3. Pin leads (PGA: Pin Grid Array) 4 vertically erected on that surface of the package 2, which faces the circuit board 15, are employed as means for connection between the wiring patterns and external circuits. A plurality of arrays of pin leads 4 are arranged along each side of the package 2. The tip ends of the pin leads 4 are connected to wiring (not shown) formed on the circuit board 15.
If the exothermic density of the semiconductor elements increases remarkably in accordance with the improvement of integration density of semiconductor elements themselves and the density of mounting of the semiconductor devices, as mentioned above, it becomes necessary to deal with the heat of the IC elements even if the amount of heat is small, in particular, in the case where the IC elements are located on that region of the circuit board, which has a low heat radiation efficiency. For example, in FIGS. 1 and 2, a wind 16 is blown from the cooling device 20 from the left to the right. The wind, which has passed by the exothermic element 11 on the circuit board 15, has absorbed heat of the exothermic element and the temperature thereof increases. Thus, the efficiency for absorbing heat of the IC elements 12 situated on the right of the exothermic element is low. Although the heat of the IC elements 12 situated on the left of the exothermic element 11 is sufficiently absorbed, the heat of the IC elements 12 situated on the right of the exothermic element 11 are not sufficiently absorbed and consequently the characteristics of the semiconductor apparatus deteriorate. Specifically, with the higher operation speed and higher integration density of the semiconductor apparatus, there may be a case where even IC elements with a power consumption lower than a predetermined power consumption of an exothermic element must be efficiently cooled in order to enhance the reliability of the entire system. In particular, in order to increase the density for mounting of elements, a memory such as an SRAM tends to be situated near a CPU where the cooling efficiency is very low. Although the generated heat must be dealt with, the heat has not yet been dealt with sufficiently.
However, it is not practical to attach a heat sink to each of semiconductor elements or to attach a heat sink commonly to all semiconductor elements of a semiconductor apparatus, since this causes an increase in cost, a decrease in mounting density, and complexity in structure.